Ovulation calculator and fertility calendar: find your best days to conceive

Last modified on Monday 29 January 2024

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Whether you're thinking about getting pregnant or are already trying to conceive, our handy ovulation calculator makes it really easy to work out when you're ovulating

Once you've narrowed the window down when you're fertile, you're one step closer to getting pregnant! Here's how it all works ...

There can be a lot to think about when you're trying to get pregnant. Luckily, when it comes to working out your ovulation window, our ovulation calculator makes life that little bit easier.

Simply enter your menstrual cycle dates, including the first day of your last menstrual period and, assuming that your cycle is pretty regular, your cycle length. For example, whether you get your period every 28, 29, 30 days – and so on.


Now click on 'calculate'.

Ovulation Calculator

Select the first day of your last menstrual period and the average length of your menstrual cycle

The results will bring up three consecutive dates per calendar month, mark them in your diary!

Why is your ovulation window important?

When you're trying to get pregnant, certain times of the month are better than others. Your ovulation window narrows down the days in your cycle when your ovary should release an egg (or eggs).

This is when you're at your most fertile and are most likely to get pregnant (assuming that you have regular sex during these dates, of course – and that there are no other factors affecting your fertility.)

The fertile window

According to the NHS:

'You're most likely to get pregnant if you have sex within a day or so of ovulation (releasing an egg from the ovary). This is usually about 14 days after the first day of your last period, if your cycle is around 28 days long.'

If your cycle isn't regular then you may need to talk to your doctor about pinpointing your ovulation window.

Other ways to do this can include:

  • Charting your basal body temperature (resting temperature). Your body temperature rises slightly just after ovulation (by around half a degree) as a result of the increase in progesterone levels. You're most fertile just before this hormone peak. Charting your temperature daily for a few months will give you a good idea of when you'll be most fertile during your next cycle. If you're charting your basal body temperature it's best to do it in the morning just after waking.
  • Using fertility sticks to monitor hormonal changes – ovulation tests or kits can be digital, electronic or come as strips. They detect the hormones that surge just before you ovulate. They can be useful tools but they're not always 100% accurate.

It's worth noting that sperm can survive inside a woman after sex for up to seven days. So you may also be able to conceive by having sex in the week leading up to ovulation – the sperm will be ready and waiting when you ovulate.

In spite of all this, many fertility experts say that you don't actually need to track your ovulation or fertile window. Studies suggest that just having sex every two or three days throughout your cycle is the best way to get pregnant for most people. But if you're having trouble conceiving, understanding your cycle can sometimes be helpful.

How can I tell when I'm ovulating?

Some women experience ovulation pain and other symptoms such as increased vaginal discharge and bloating when they're ovulating. Although not every one will notice these, here's more about the signs and symptoms of ovulation.

Aa mentioned above, another way to tell when you're ovulating is by using ovulation kits. These measure the amount of hormone in your urine to tell you when you're at your most fertile. There are 2 type hormones we use to predict ovulation.

Estrogen is made by the growing egg. It rises early in your monthly cycle before you ovulate. Rising estrogen levels means your body is getting ready to release an egg.

Luteinising Hormone (LH) is produced in response to the estrogen and in the 24 to 36 hours before ovulation, it surges. That makes it the best predictor of ovluation.

By using an ovulation test that measures both – such as the Clearblue® Advanced Digital Ovulation Test – you'll get accurate results that really zero in on the days you're most likely to get pregnant.

The Clearblue Advanced Digital Ovulation Test typically identifies 4 or more fertile days each cycle1 and tells you when estrogen is high as well as when you have the surge of LH. Not only that, it's easy to read, with a smiley face in the results window. That means no guessing at the best time to get pregnant!

Find the best ovulation kits here.

Understanding your cycle and phases of the menstrual cycle

Before we start to think about getting pregnant, many of won't think much about our cycle other than when our period starts.

But when it comes to TTC, understanding how your cycle works can be really helpful.

The average menstrual cycle is 28 days long and divided into three different phases known as the follicular phase (14 days including your period), the ovulation phase (one day) and the luteal phase (14 days). The follicular phase, in particular, can vary in length, depending on how long your cycle is.

Your cycle starts on the first day of your period (day 1).

Ovulation happens around 12-16 days before the first day of your next period.

Once your ovary releases an egg, it travels along your fallopian tube where it needs to be fertilised within 12-24 hours of it being released.

If this happens, then voila – you're pregnant!

Now keep reading to find out more about the different weeks of an average 28 day cycle ...

Woman holding Clearblue Ovulation Test

Your cycle: week one

Day one of your cycle is the day your period starts. The follicular phase starts at the same time.

If you're lucky enough to conceive during this cycle, then this will become week one of your pregnancy, because pregnancy weeks are calculated from the first day of your last period before you became pregnant.

If you're TTC and your period is late, it's time to take a pregnancy test!

However, if you weren't so lucky to conceive this time, keep trying and make sure that you maximise your chances of getting pregnant later on in your cycle by:

Make sure your partner knows what he can be doing to help you get pregnant, too (besides the obvious!)

Find out more here.

Your cycle: week two

This week you're a step nearer to ovulation and the most fertile part of your cycle. The follicular phase is in full swing, and an egg (or eggs) is maturing in your ovaries in preparation for ovulation.

Remember, that your ovulation window is only a rough guide so there's never any harm in practising right through your cycle.

In fact, according to NHS advice:

'If you want to get pregnant, having sex every 2 to 3 days throughout the month will give you the best chance.'

Find inspiration here!

After all, when you have sex, sperm can live inside your body for up to seven days. So even if your ovary hasn't released an egg yet, it's possible to have sperm ready and waiting to fertilise it when ovulation does happen.

Your cycle: week three

Ovulation usually happens around now, which means you're officially at your most fertile. So, if you're trying to get pregnant, now is definitely the time to have regular sex.

And don't worry about lying still or having your legs up in the air, either. It's an old wives' tale – although it certainly won't do any harm if you do happen to do it!

After ovulation, your luteal phase begins.

Your cycle: week four

You're heading out of your most fertile stage and it's quite possible that conception has happened. Once the egg has been fertilised, it'll be implanted in the thickened lining of your womb this week.

If you've not been lucky this time, then your body will be preparing to shed the unfertilised (tiny) egg along with the lining of your womb and you'll get your period at the start of next week.

That takes you back to the start of the cycle.

Of course, if you don't get your period then you'll soon be able to take a pregnancy test and confirm whether you're pregnant or not.

If you are, the next thing you'll be needing is our pregnancy due date calculator ... see below!

Whether or not you're pregnant this time, you'll always find other mums on the same journey as you in our forum.

Due date calculator

Select the first day of your last menstrual period and the average length of your menstrual cycle

Are you TTC? Get support on your journey in our friendly forum, or check out our articles below for more info.

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